Epoxy resin has high heat resistance, excellent adhesive properties, high water resistance, high mechanical strength, and excellent electrical characteristics and is therefore used in various fields, such as adhesives, paints, civil engineering and construction materials, and insulating materials for electrical and electronic components. In particular, in electrical and electronic fields, epoxy resin is widely used in insulating casting, laminated materials, sealing materials, and the like.
In printed wiring boards for use in electrical and electronic devices, particularly multilayer printed wiring boards, with recent reductions in the size and weight and increases in the functionality of devices, there are demands for further increases in the number of layers and packaging density, reductions in thickness and weight, and improvements in reliability and molding processability.
Thus, as wiring becomes finer, among the characteristics of resins serving as substrate materials, there are particular demands for further improvement in heat resistance and reduction in the coefficient of linear expansion.
At present, one of low linear expansion thermosetting resins is polyimide resin. Polyimide resin is the mainstream in the field of flexible printed wiring boards. However, polyimide resin is difficult to process because of its high heat resistance. Furthermore, polyimide resin is very difficult to handle during processing because polyimide resin dissolves only in particular polar solvents. Thus, polyimide resin has various limitations. For example, hitherto known imide skeleton compounds have insufficient solubility in common organic solvents, such as ketone solvents, and dissolve only in polar solvents, such as dimethylformamide and N-methylpyrrolidone (Patent Literature 1).
In contrast, epoxy resin has better processability and higher solubility in solvents than polyimide resin. However, epoxy resin is unsatisfactory with respect to the coefficient of linear expansion.
Although an anthracene skeleton epoxy resin, a bisphenol S skeleton epoxy resin, and a naphthalene skeleton epoxy resin are conventionally known as epoxy resins having a low coefficient of linear expansion, these resins have still higher coefficients of linear expansion than polyimide resin.
Patent Literatures 2 and 3 describe imide skeleton epoxy resins that are structurally different from a soluble imide skeleton resin according to the present invention.
Patent Literature 1 relates to a thermoplastic polymer produced by the reaction between imidophenol and an epoxy resin. This thermoplastic polymer dissolves in polar solvents, such as dimethylformamide and N-methylpyrrolidone, but does not dissolve in ketone solvents. Patent Literature 4 also describes a composition produced by the reaction between imidophenol and an epoxy resin. This resin has low solubility in a polar solvent N-methylpyrrolidone, and it goes without saying that the resin does not dissolve in ketone solvents.